1. Field of the Invention
The present invention relates to means for dividing and recombining light from an achromatic light source. In particular, it relates to such means as may be employed to provide an improved optical block for a full color projection system such as that of the above-referenced United States patent.
2. Description of the Prior Art Including Prior Art Statement
An efficient and relatively compact high brightness full color projection system is disclosed in U.S. Pat. No. 4,127,322. That system, which represents a significant improvement over prior art systems discussed therein, modulates a high intensity projection light source with a relatively low intensity primary triad of component CRT images, the sum of which is the full color image to be projected. Polarization modulation is achieved by the reflection of monochromatic light from any one of the three a.c. field effect light valves, each of which is associated with one monochromatic CRT. A single MacNeille biprism is utilized to polarize the incoming projection light while an array of wavelength-selective dichroic filters relays the light to the proper light valve for modulation thereof. Acting as an analyzer, the biprism receives the (modulated) projection light reflected from the three light valves. This light, being polarized along an axis before reflection, will contain an oppositely polarized component which is proportionate to the phosphor intensity of the (appropriate bandwidth) CRT associated with each light valve. The biprism simultaneously reflects the s-polarized component of the reflected (modulated) light to separate projection optics for each of the three (red, green and blue, for instance) full color component bandwidths. The p-polarized component of each passes through the biprism, returning to the light source.
An inherent limitation of the above system results from the known wavelength-dependence of biprism extinction ratio (ratio of the s-polarized, modulated light to the p-polarized unmodulated light being directed to the projection optics). It is desirable that the above-named ratio be as high as possible. For a system of the above-described character, the wavelength dependence of the single analyzer-polarized biprism results in an amount of unmodulated light in the projected image as a wavelength-dependent variation exists in the proportion of modulated (s-polarized) light reflected at each frequency and the proportion of the unmodulated (p-polarized) light not reflected at each frequency.